The invention relates to solid oxide fuel cells and more particularly to a method of fabricating the fuel cell core. A fuel cell is basically a galvanic conversion device that electrochemically reacts a fuel with an oxidant within catalytic confines to produce a DC electrical output. In a fuel cell, cathode material defines the passageways for the oxidant and anode material defines the passageways for the fuel, and an electrolyte separates the cathode and anode materials. The fuel and oxidant fluids, typically as gases, are continuously passed through the cell passageways separated from one another. The fuel and oxidant discharges from the fuel cell generally remove the reaction products and heat generated in the cell. The fuel and oxidant are the working fluids and as such are typically not considered an integral part of the fuel cell itself.
The type of fuel cell for which this invention has direct applicability is known as the solid electrolyte or solid oxide fuel cell, where the electrolyte is in solid form in the fuel cell. In the solid oxide fuel cell, hydrogen or a hydrocarbon fuel is preferably used as the fuel and oxygen or air is used as the oxidant, and the operating temperature of the fuel cell is between 700.degree. and 1,100.degree. C.
The hydrogen passing through the fuel cell reacts with oxide ions on the anode to yield water, which is carried off in the fuel flow stream, with the release of electrons into the anode material. The oxygen reacts with the electrons on the cathode surface to form the oxide ions which then pas into the electrolyte material. Electrons flow from the anode through an appropriate external load to the cathode, and the circuit is closed internally by the transport of oxide ions through the electrolyte. The reaction process is well known and more thoroughly delineated in U.S. Pat. Nos. 4,499,663 and 4,816,036.
The electrolyte isolates the fuel and oxidant gases from one another while providing a medium allowing oxygen ion transfer, as well as voltage buildup on opposite sides of the electrolyte. Fuel and oxidant must diffuse away from the flow stream in the respective passageways to the electrolyte and react at or near the boundary of the electrodes (anode or cathode), and electrolyte, where electrochemical conversion occurs. The electrodes provide paths for the internal movement of electrical current within the fuel cell to the cell terminals, which also connect with an external load. The operating voltage across each cell is on the order of 0.7 volts so the individual cells must be placed in electrical series to obtain a useful load voltage.
U.S. Pat. No. 4,476,198 (Ackerman, et al) discloses a monolithic core consisting only of materials active in the electrochemical reactions. The thin composite core walls are shaped to define small passageways. If the core walls are constructed without deformation, they are predicted to have sufficient structural integrity to withstand the fluid pressures generated by gas flow through the passageways and the mechanical stresses due to the weight of the stacked core walls on one another. This monolithic construction would beneficially increases the power density of the fuel cell because of its reduced size and weight.
U.S. Pat. No. 4,476,196 (Poeppel, et al) discloses a monolithic core construction having the flow passageways for the fuel and for the oxidant gases extended transverse to one another. The core construction provides that only anode material surround each fuel passageway and only cathode material surround each oxidant passageway, each anode and each cathode material further being selectively sandwiched at spaced opposing sides between electrolyte and interconnect materials. These composite anode and cathode wall structures are alternately stacked on one another (with the separating electrolyte or interconnect material typically being a single common layer) whereby the fuel and oxidant passageways are disposed transverse or in a cross flow relationship to one another.
U.S. Pat. No. 4,510,212 (Fraioli) discloses a core construction having both parallel and cross flow paths for the fuel and the oxidant gases. Each interconnect wall of the cell is formed as a sheet of inert support material having therein spaced small plugs of interconnect material, the cathode and anode materials being formed as layers on opposite sides of each sheet and being electrically connected together by the plugs of the interconnect material. Each interconnect wall is formed into a wavy shape and then connected along spaced, generally parallel, line-like contact areas between corresponding spaced pairs of generally parallel electrolyte walls, operable to define one tier of generally parallel flow passageways for the fuel and oxidant gases. Alternate tiers are arranged to have the passageways disposed normal to one another.
Cellular type fuel cell cores (see U.S. Pat. No. 4,476,198) of the prior art are made by the process whereby the compositions used for the four materials are put into four distinct slurries. Each slurry is then placed in a reservoir of a squeegee-type device which is pulled over a flat surface and hardens or plasticizes into a layer of the material having the desired thickness. In this manner the electrolyte wall or interconnect wall is formed by a first layer of anode material followed by a layer of either electrode or interconnect material and finally by a layer of the cathode material. The layers are bonded together since the binder system is the same in each layer.
Related U.S. Pat. No. 4,816,036 (Kotchick) teaches another method of forming a cellular core, whereby the compositions for the four materials are individually mixed to a plastic consistency and subsequently hot rolled into thin sheets. The thin sheets can then be hot rolled into multilayer tapes, formed, stacked, and fired as a monolith to produce the fuel cell with integral fuel and oxidant manifolding.
Theoretically, the fuel cell stack structure of the above-noted patents should provide exemplary power density. However, problems arise because the fuel cell stacks are formed from arrays of anode and cathode sandwiching either an electrolyte or interconnect material in pliant or green form. The various green constituent layers are stacked on top of each other, appropriately oriented, to form the stack structure. The resultant stack structure is made up of all green, or unsintered, constituent parts. As noted, the components of the stack are made of different materials, and thereby requiring one to try to match the coefficient of thermal expansion and firing shrinkage for the different materials as closely as possible to one another to minimize separation problems. Fuel cell stacks which are made of green precursors which are all cosintered display poor performance due to microcracks which occur in the various layers as a result of the cosintering step if thermal expansion and firing shrinkage matched is not achieved. A resulting cosintered stack produces significantly less current than its theoretical current density due to mixing of reactant gases which is the direct result of the micro-cracks in the stack. In addition, it is difficult to densify the interconnect under conditions suitable for the other cell components. Inadequately densified interconnects allows cross-leakage of reactant gases.
A second problem arises when the multilayer fuel cell stacks of the prior art are densified in that there is migration of the ceramic materials, primarily the interconnect material, into adjacent layers. This migration of the ceramic materials adversely effects the resulting component physical properties of density, porosity, and homogeneity. A third problem is the slumping of the corrugations during co-firing. As the size of the fuel cell structure increases the green corrugated layers are not stiff enough to support the structure weight during the heat treatment process. Accordingly, a processing method and the resulting fuel cell which eliminate the problems of microcracks, ceramic migration, and slumping would be desirable.